With the growth in the consumer printer market, inkjet printing has become a broadly applicable technology for supplying small quantities of liquid to a surface in an image-wise way. Both drop-on-demand (DOD) and continuous inkjet (CIJ) devices have been conceived and built. Whilst the primary development of inkjet printing has been for aqueous based systems with some applications of solvent based systems, the underlying technology is being applied much more broadly.
Inkjet printing continues to strive for higher spatial resolution. Because the colorant is uniformly distributed within the ink at a constant concentration in all current systems (DOD and CIJ), this resolution is determined by the droplet size. Thus in order to push to higher resolution, it will be necessary to generate smaller drops at higher frequency. In producing ever-smaller drops, several technical constraints become progressively more severe.
First, ever higher droplet velocity is required to maintain droplet inertia and therefore throw distance and placement accuracy. Second, to produce smaller drops smaller orifices are required. This then demands finer filtration since the nozzles will block more easily. Third, the smaller nozzle diameter will require higher back pressure to ensure the same jet Weber number.
The break-up of a thin liquid jet driven by capillary forces is well known. The instability is driven by capillary forces that reduce the surface area of the jet by forming droplets, and is known as the Rayleigh-Plateau instability (J. Plateau 1873; see e.g. T. E. Faber, “Fluid dynamics for physicists”, CUP 1995, p 295 or P. G. de Gennes, F. Brochard-Wyart, D. Quéré, “Capillarity and Wetting Phenomena”, Springer 2004, p 118.). The process is seen in nature in the dew that forms on a spider's web, where the uniform film on each thread eventually beads up to form separated droplets. It is also routinely seen in the dripping of a household tap. In recent years this instability has been widely studied in the context of inkjet printing. For drop-on-demand (DOD) printing, there is usually a thread of liquid that follows drop ejection and which subsequently disintegrates to form unwanted satellites. For continuous inkjet (CIJ), the continuous formation of droplets from the jet in a controlled fashion is fundamental to the robust operation of the process. It is well understood that the growth of random perturbations to the jet (radius, pressure, velocity, surface tension etc.) lead to the formation of drops at an average frequency corresponding to approximately 9 times the jet radius, the so called Rayleigh frequency. It is also well known that by periodically perturbing the jet the drop formation can be stabilised at frequencies between approximately 0.25×Rayleigh frequency and 1.25×Rayleigh frequency. At frequencies above approximately 1.39×Rayleigh frequency, the jet is in fact stable. Although most attention has been paid to liquid jets in air, exactly the same process takes place for an immiscible jet in another liquid, where it is now the interfacial tension that is the driving force. Again the Rayleigh frequency is observed, though the final break-up process is slightly different in detail.
A new continuous inkjet device based on a MEMs formed set of nozzles has been recently developed (see U.S. Pat. No. 6,554,410). In this device a liquid ink jet is formed from a pressurized nozzle. One or more heaters are associated with each nozzle to provide a thermal perturbation to the jet. This perturbation is sufficient to initiate break-up of the jet into regular droplets through the Rayleigh-Plateau instability. By changing the timing of electrical pulses applied to the heater large or small drops can be formed and subsequently separated into printing and non-printing drops via a gaseous cross flow. All continuous inkjet processes require capture and recirculation of non-printing droplets. These processes are problematic for liquids containing many useful additives such as polymeric materials and colloidal materials such as pigments. Furthermore, because of the need to sort printing droplets from non-printing droplets, contone printing is not in general possible, i.e. all printing droplets being the same size and the same colorant density. By enabling contone printing the requirement for high spatial resolution is significantly reduced since with a binary printing arrangement spatial resolution is used to create the illusion of colour variation.
In the last several years the field of microfluidics has grown significantly. Inkjet drop generation devices are microfluidic devices in that they employ very small scale liquid channels. The implication of this is that the Reynolds number
  Re  =            ρ      ⁢                          ⁢      UL        μ  where ρ is the liquid density (kg/m3), U is a characteristic velocity (m/s), L a characteristic length (m) and μ the liquid viscosity (pa·s), is sufficiently small that inertial effects are sufficiently small that the flow is predominantly laminar in nature. For a typical continuous inkjet system the velocity might be 20 m/s and a length might be 5 μm with a density approximately 1000 kg/m3 and a viscosity of 1 mPas, the Reynolds number is therefore approximately 100. The transition to turbulent flow in a straight pipe occurs at Re above approx 2000.
Microfluidic devices where the liquid flow is laminar necessarily prevent mixing. In fact the only mechanism available for mixing is diffusional flow. For example, consider a T junction in which two fluids are injected to flow alongside each other. How far down the channel must the fluids flow before the channel is homogenized? A simple estimate requires the particles or molecules to diffuse across the entire channel, giving a time tD˜w2/D, where w is the width of the channel and D is the diffusion constant. During this time, the material will have moved a distance Z˜U0w2/D down the channel, so that the number of channel widths required for complete mixing would be of order
      Z    w    ≈                    U        0            ⁢      w        D    ≡  Pe
The dimensionless number on the right is known as the Péclet number (Pe), which expresses the relative importance of convection to diffusion. In this example, the number of channel widths required for full mixing varies linearly with Pe. Using the diffusivities in the table below, estimated using the Stokes-Einstein relation, we see that even a dye molecule flowing with the fluid through a 10 μm channel at 1 m/s requires Pe ˜250000 channel widths to completely mix. Alternatively, that same dye molecule flowing with the fluid at 1 m/s would require a pipe length z˜25 mm to diffuse 1 μm.
Characteristic Diffusivities in water at room temperatureParticleTypical sizeDiffusion constantSolute ion10−1nm2 × 103μm2/sDye molecule5nm40μm2/sColloidal particle100nm2μm2/sBacterium1μm0.2μm2/sMammalian/human cell10μm0.02μm2/s
A class of microfluidic device that has recently proved extremely interesting are flow focussing devices (FFD see e.g. Anna et al Appl Phys Lett 82, 3 (2003) 364; US 2005/0172476). In an FFD a liquid flows into a middle channel and a second immiscible liquid flows into one or more outside channels. The two liquid phases are then forced to flow through a small orifice that is located downstream of the channels. The outer fluid exerts pressure and viscous stresses that force the inner fluid into a narrow thread, which then breaks inside or downstream of the orifice. These devices are of interest because by operating in either a geometry controlled or dripping mode, monodisperse droplets are formed that have many uses, e.g. emulsion formation, drug encapsulation, particle engineering etc. However, monodisperse drops are not formed in the jetting regime, i.e. where the central immiscible liquid breaks up via capillary forces in the Rayleigh regime.
An alternative droplet formation device brings two immiscible liquids together at a T junction (WO 2002/23163). In this device the shear of the outer fluid on the interface of the inner liquid creates the droplets in a dripping mode. The size of the droplet so formed is controlled by the ratio of the shear stress acting on the liquid-liquid interface and the interfacial tension.
Using these devices various operations on droplets can be performed (see US 2005/0172476, US 2006/0163385, US 2006/0234051, US 2007/0054119, WO 2004/091763). In general the purpose is to engineer droplets or particles or to encapsulate, for example, drugs. Coalescence of droplet streams is achieved (US2007/0003442), mixing and polymerisation of droplets (WO2005/103106), and multiple emulsions are formed (WO2006/096571).
EP 1364718 discloses a method of generating encapsulated droplets via co flowing immiscible liquids. In this method the liquids are supplied by coaxially arranged nozzles, which are difficult to manufacture as an array. Further, this method relies on a strong electrostatic field to ensure break-up of the coaxially arranged liquids.
JP 1996207318 also uses coaxial tubes and electrostatics to break off a droplet. The centre tube in this case can supply colloidal particles or a plurality of them to provide a colour level. Electrophoretic means can stop the flow of particles by arrangement of electric fields.
WO 2006/038979 describes a concentric piezoelectric system to enable encapsulated drop on demand printing.
U.S. Pat. No. 6,713,389 describes placing multiple discrete components on a surface for the purpose of creating electronics.
U.S. Pat. No. 5,113,198 describes using a carrier gas stream to direct vaporous dyes toward a surface. It discloses use of co flowing gas streams, but not liquids.
U.S. Pat. No. 6,377,387 describes various methods for generating encapsulated dispersions of particles.
WO 2002/23163 describes cross-flow devices for making emulsion droplets for biological applications.